Allow transform callback to return a promise

As part of the `async` API it'd be great to return a promise from the `transform` method. The reason is to allow usage of postcss plugins that are async, like 3.x of `stylelint`.

``` js
transform(css, filename) {
  return processor.use(stylelint).process(css, {from: filename});
}
```

Unless there's a way to do something like this already?
